Water-deficient Calc-alkaline Plutonic Rocks of Northeastern Superior Province, Canada: Significance of Charnockitic Magmatism |
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| Authors: | PERCIVAL JOHN A; MORTENSEN JAMES K |
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| Institution: | 1GEOLOGICAL SURVEY OF CANADA, 601 BOOTH STREET, OTTAWA, ONTARIO, K1A 0E8, CANADA
2DEPARTMENT OF EARTH AND OCEAN SCIENCES, UNIVERSITY OF BRITISH COLUMBIA, 6339 STORES ROAD, VANCOUVER, B.C., V6T 1Z4, CANADA |
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| Abstract: | Calc-alkaline batholiths of the Archaean Minto block, northeasternSuperior Province, Canada, have pyroxene- and hornblende-bearingmineral assemblages inferred to have crystallized from hot,water-undersaturated magmas at 2·729–2·724Ga. A regional amphibolite- to granulite-facies tectonothermalevent at 2·70 Ga resulted in mild to negligible metamorphiceffects on the dominantly granodioritic units. Geochemical,textural and thermobarometric studies define the crystallizationhistory in compositions ranging from cumulate pyroxenite throughquartz diorite, granodiorite, granite, and syn-magmatic gabbroicdykes. Early magmatic assemblages include orthopyroxene, clinopyroxene,plagioclase, biotite, Fe–Ti oxides and ternary feldspar,indicating crystallization from magmas containing <2 wt %H2O at 1100–900°C. Water enrichment in the residualmelt induced hornblende crystallization at 5 ± 1 kbar,800–600°C. Characterized by a continuum of large ionlithophile element (LILE)-enriched, high field strength element(HFSE)-depleted compositions, the I-type suite resembles moderncontinental arc batholiths in composition and size but not primarymineralogy. Magmatic arcs produced between 2·75 and 1·85Ga commonly have charnockitic components, possibly because slab-derivedfluids interacted with mantle wedges at ambient temperatureshigher by  100°C than at present, producing large volumesof water-deficient magma. KEY WORDS: granitoid rocks; igneous pyroxenes; water-undersaturated magma; charnockite |
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